Kinosternoidea
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Kinosternoidea is an obsolete superfamily of aquatic turtles, which included two families: Dermatemydidae (Mesoamerican River Turtles), and Kinosternidae (American mud and musk turtles). Sometimes, even the entirely uznrelated Big-headed Turtle (Platysternon megacephalum) was included with the "kinosternoids" too.
These are nowadays usually considered independent families of the Trionychoidea, among which they represent very plesiomorphic members which share a few peculiarly advanced traits. These apomorphies coupled with the overall "primitiveness" was what mislead scientists as to their actual relationships.